1. Field of the Invention
This invention relates to the field of optical spatial filtering. More particularly, this invention concerns a system in which the spatial filtering characteristics are dynamically controllable with a high degree of versatility and precision.
2. Description of the Relevant Art
Systems which project an optical image on a surface have a wide variety of applications. These applications include microscopy and photolithography. In these areas, a high degree of resolution and dynamic adaptability are essential. One method for increasing image resolution is known as spatial filtering.
A variety of changes can be made in the clarity, resolution, and appearance of the details in an optical image by modifying (filtering) the amplitude or phase of the light in a variety of regions of the specimen's spatial Fourier transform. The spatial Fourier transform is produced most accurately if the light source has a small size in terms of its subtended angle. Maximum spatial-frequency discrimination is obtained in spatial filtering if the light is highly coherent both spatially and temporally. Light from a point source is highly coherent spatially, and monochromatic light is highly coherent temporally. Both types are useful in spatial filtering. Furthermore, both types are easily obtained through use of a laser beam focused by a lens through a pinhole.
The characteristics of the Fourier spectrum are predictable from the spatial distribution of amplitude transmittance and phase in the image bearing medium. The amplitude transmittance characteristics of the image-bearing medium can be resolved by Fourier analysis into its sine wave and cosine wave components, each of which has a certain spatial frequency, amplitude, azimuth orientation, and phase. Each point in the spectrum represents, for any given wavelength of light, a particular spatial frequency in the transparency. The intensity at each point in the spectrum is proportional to the square of the amplitude of the corresponding spatial frequency component of the transparency. The high frequency and edge sharpness information is represented, in the Fourier transform plane, by points of light that lie at a greater distance from the optic axis than the points of light representing the low frequency information and large area contrast of the transparency.
For the purpose of increasing the edge sharpness and fine-detail contrast of the projected image of a photographic transparency, a certain type of spatial filter can be inserted into the Fourier transform plane. Such a filter can comprise a clear, optically flat, glass plate bearing a light-absorbing medium that in some pre-selected pattern (pattern of dots, variable density areas, etc.) transmits a fraction (such as one-fourth in terms of intensity or one-half in terms of amplitude) of the light in the zero-frequency and low-frequency regions but transmits progressively more freely in the medium and high-frequency regions.
Spatial filtering can be done with either coherent or partially coherent light, but the spatial frequency discrimination is greatest when the light is highly coherent. If in the particular application of interest, high discrimination is not required, the degree of coherence can be substantially reduced as a means of increasing the light intensity. The effective source is the illuminated pinhole, and if the size of the pinhole is increased and/or the wavelength bandwidth of the light is increased, the spatial frequency discrimination can be reduced to its minimum required value. With partially coherent light, the spatial filtering is always gradual with respect to spatial frequency. Abrupt changes with respect to spatial frequency are possible only with highly coherent light, but the gradual type is satisfactory in many practical applications. A decrease in coherence is, with most sources, accompanied by a large increase in intensity, which can be an important practical advantage.
One advantage of optical spatial filtering is that the resolution of the spatial filter is not critical. In fact, beam manipulation at the Fourier plane may take place at a much larger scale than that of the image source.
Spatial filtering is typically implemented using a trial-and-error approach. A projected image is studied, flaws determined, and various corrective approaches are attempted. The corrective approaches often consist of attempting various combinations of standard patterns for spatial filtering until the desired result is achieved. This approach is practical only because a limited number of identified flaws and filtering solutions are dealt with.
It is then desirable to provide a dynamically adjustable system which can not only implement the pattern combinations described above, but also provide a method of fine-tuning the combinations for specific source images and image plane characteristics. An additionally desirable characteristic of such a system would be the ability to search for novel filtering solutions to identified flaws.